Corrugated board generally refers to a composite laminated structure comprising two paper linerboard sheets bonded to a paper corrugated or fluted core material. In the context of the present description a corrugated material will mean at least one liner sheet bonded to at least one fluted layer. Single faced corrugated board refers to the structure where there are single sheets of linerboard bonded to the tips of a single fluted core layer. There are various other combinations which can be fabricated to form various well known products such as double wall board, triple wall board, and the like.
The typical process for forming corrugated materials is well known. Typically the single wall corrugated board will be formed on an in-line corrugator where large rolls of linerboard paper and a large roll of the medium paper (the starting raw material for forming the corrugated core layer) will be positioned at the upstream end of the corrugator. The process normally has the medium passing through a set of corrugating rolls and thereafter being bonded to the first liner as it travels in a downstream direction. A suitable adhesive is utilized to attach the flute tips on one side to the inside surface of the first liner. Thereafter, at a downstream location, the other liner material is laminated to the other flute tips with a suitable adhesive to form the combined board. The combined board then travels into a heating and cooling section to allow the adhesive to fully cure and bond the liners to the fluted medium. Immediately thereafter, slitter-scorer and cutoff devices function to produce individual flat sheets of slit and scored blanks for converting into containers.
As is well recognized by those skilled in the art, the structural requirements for combined board are determined by the particular requirements of the end use. Standards have been developed over the years and both the liner and medium materials are manufactured to preselected basis weights with the end-use requirements in mind. In a container a particularly important property is vertical stacking strength (top to bottom) where the vertical walls in a container are expected to support in-use compression loadings. In almost all packaging end uses the cross machine direction of the fluted medium will be oriented in a vertical direction in the package side walls. In this orientation, the liners and fluted medium will provide good vertical stacking strength. Another strength property relates to the crushing resistance (flat crush) of the combined board in a plane perpendicular to the plane of the combined board. This property depends upon the strength of the corrugated medium.
The vertical stacking strength for a container depends to a large extent upon the basis weights of the component materials. With higher basis weights for the component materials the higher the vertical stacking strength and normally the greater the top-to-bottom crush resistance. When a package needs greater stacking strength, higher basis weights will be utilized. Normally it is the customer that specifies the strength requirements and the corrugated board manufacturer will produce the packaging material accordingly. A typical combined board with medium strength is one that utilizes liner sheets with a basis weight of 42 pounds per thousand square feet and with a medium material of a basis weight of 33 pounds per thousand square feet (MSF). Typically, a package comprised of this combined board will result in an edgewise compression test (ECT) or short column value of about 45 pounds per lineal inch. A higher stacking strength can be provided by using liner sheets with a basis weight of 69 pounds per MSF and a medium basis weight of 33 pounds per MSF. This gives an ECT of about weights, additional fiber must be utilized which adds to the cost of producing the combined board and the resulting container.
Ways have been sought for many years to increase the vertical stacking strength and/or flat crush resistance without the need for utilizing additional fiber. One suggested approach has been to utilize resin compounds sprayed or otherwise coated onto one or both of the liner sheets or onto the medium material before it is fluted. In the past, isocyanate compounds have been suggested as a suitable resin which, after curing, will add strength to the particular component of the combined board. Most of these chemical additives and their processing requirements have not resulted in commercially viable strength enhancing processes and products. Problems revolve around resin handling, safety issues, costs, recycling and the like.
While the addition of resin is known for increasing the strength of packaging materials, one of its inherent problems is that once the resin cures it tends to yield a paper that is brittle and relatively inflexible. After the combined board is produced and cut into flat sheets, then the package itself must be erected. This commonly requires a plurality of slitting, scoring and folding steps which presents problems for a sheet material that is relatively brittle. One of the problems with resin additions has been the difficulty in forming good containers with treated corners that do not fracture.
The inventors of the present invention has developed a process for enhancing the strength of corrugated board packaging materials that impacts the aforementioned problems. Relatively low amounts of selected isocyanate compounds have been found to provide significant strength enhancement in the combined board when applied according to the steps of the present invention. In addition, the resulting board made in accordance with the processes of the present invention has proven to be readily recyclable to facilitate reclamation of pulp fibers from used articles made from the boards.
Accordingly, from the foregoing, one object of the present invention is to enhance the strength characteristics of corrugated board packaging material.
A further object of the present invention is to utilize relatively small amounts of isocyanate resin compounds for the strength enhancing purpose.
Yet another object of the present invention is to increase both short column strength (top to bottom) and flat crush resistance in a combined board packaging material without utilizing more fiber.
An additional object of this invention is to provide a strengthened packaging material that has the same characteristics of existing packaging materials in areas such as surface friction, white top compatibility, color and repulpability for recycling purposes.
Still another object is to minimize the contamination of the corrugator equipment with the strength enhancing resin by applying the liner adhesive and liner essentially immediately after the resin is applied to the fluted medium.
Another object of the present invention is to provide a strength-enhanced corrugated paperboard that is readily repulped.
Still another object of the present invention is to use the repulped, strength-enhanced paperboard to produce recycled paper products.
These and other objects of the present invention will be better understood upon reading the specification which follows in conjunction with the attached drawings.